The metabolic fate of Nipradilol (K-351 : NIP), a new potent antihypertensive and antianginal agent, was investigated in the dog and human. From dog urine, the parent drug and 11 metabolites were isolated by column chromatography on Amberlite XAD-2 resin and silica gel. Their chemical structures were deduced from spectral comparisons (mass, proton and carbon-13 nuclear magnetic resonance, infrared) with synthetic samples, and quantitative determination was accomplished by gas chromatography-mass spectrometry (GC-MS) (selected ion monitoring method). NIP was metabolized by four principal pathways in the dog : (a) denitration, giving denitro NIP ; (b) aliphatic and aromatic hydroxylation of the 3, 4-dihydro-2H-1-benzopyran ring, giving 4- or 5-hydroxy NIP or denitro NIP ; (c) degradation of the isopropylaminopropanol side chain, including N-deisopropylation followed by N-methylation or deamination ; (d) glucuronidation. 4-Hydroxy metabolites had a trans-conformation with 3-and 4-substituents in a pseudoaxial position. In the human, 7 metabolites out of the 11 were identified ; aromatic hydroxylation and N-methylation were not found, suggesting the existence of pronounced species differences. Moreover, degradation of the side chain was a minor route among the above pathways.